The present invention relates to forming short light pulses that can be used in particular for transmitting data over an optical fiber. It is known that the combined action of dispersive optical effects and of Kerr-type non-linear effects that are imparted by such a fiber enables suitably-shaped pulses referred to as "solitons" to propagate without being deformed. That is why using such pulses is being considered for high-rate transoceanic transmission (6,000 km-9,000 km). Such long transmission distances can be achieved by propagating solitons along fibers in which losses are compensated by optical amplifiers having erbium-doped fibers.
Soliton transmission requires a train of periodically-emitted short light pulses. For example, for 10 Gbit/s transmission, each pulse must measure about 20 ps (at half-intensity). Furthermore, the pulses must be "close to the Fourier limit", i.e. a critical product which is the product of the duration of a pulse multiplied by its spectrum width must be less than a limit of about 0.7, and must be as close as possible to an optimum value of 0.32.
In a first known method, such pulses can be generated by mode coupling in a semiconductor laser having an external cavity. That method is described by D. M. Bird, R. M. Fatah, M. K. Cox, P. D. Constantine . . . , Electronics Letters 26, p. 2086 (1990). A second known method uses gain switching in a monolithic semiconductor laser, and is described by a Document by Downey: L. M. Downey, J. F. Bowers, R. S. Tucker and E. Agyekum, IEEE, J. Quant. Elec. QE 23, 1039 (1987).
The first method requires a complex laser which is difficult to use. It would therefore be difficult to consider using that method on site.
The second method makes it possible to emit pulses which have very wide spectrum widths which means they are far from the Fourier limit. Such spectrum widths result from the fact that laser gain switching is obtained by injecting a current that varies considerably. The optical frequency then varies considerably during each of the pulses produced because of the fact that the refractive index of the material making up the laser cavity depends on the current. Such variation in the refractive index results in a corresponding variation in the optical length of the laser cavity, and therefore in the resonance optical frequency of the cavity during the delivered pulse. As a result, the pulse has its rising edge formed by a wave of higher optical frequency than that of its falling edge (FIG. 2).
That variation in frequency may be used to reduce the duration of the pulse by using a known "compression" method. In that method, the pulse passes through an optical medium whose optical path length depends on optical frequency. In particular, a special fiber having highly offset dispersion (in which the wavelength that cancels the dispersion is greater than the central wavelength of the pulse emitted by the laser) has a longer optical path length for high frequencies than for low frequencies.
The variation in frequency therefore enables the rising edge to be delayed relative to the falling edge: by adjusting the length of the fiber, a pulse is formed whose corrected duration is much shorter than its initial duration, and whose critical product is therefore close to the desired ideal value.
In particular, a known DFB laser delivers pulses, each of which has a critical product of about 0.75 when compressed.